Radiation hard circuits are required for space and military applications. Generally, Silicon-on-Insulator Complementary Metal Oxide Semiconductor (SOI CMOS) devices are insensitive to transient radiation. They are, however, sensitive to total dose radiation effects.
The primary factor limiting the total dose radiation hardness of SOI CMOS devices is the excessive threshold voltage shift that occurs with increased radiation dosage. Ordinarily, the threshold voltages of both PMOS and NMOS devices increase negatively, and at the same rate of change, as the total radiation dose increases. When CMOS devices are incorporated in digital circuits, the effect of such threshold voltage shifts, most often, is that it becomes impossible to turn the circuits "on" or "off" using the normal signal voltages.
One method for increasing the radiation resistance of MOS devices is described in U.S. Pat. No. 4,748,131, issued May 31, 1988, to T. C. Zeitlow, titled "Method for Increasing Radiation Hardness of MOS Gate Oxides". In the method of the '131 patent, fluorine is introduced into the gate oxide by robust procedures which result in a significant reduction of radiation induced interface state density, thereby producing enhanced radiation hardened MOS devices.
U.S. Pat. No. 4,866,498, issued Sept. 12, 1989, to D.R. Myers, titled "Integrated Circuit with Dissipative Layer for Photogenerated Carriers" discloses a CMOS integrated circuit in which the conventional silicon dioxide layer supporting the active devices on a semiconductor substrate is replaced by a dissipative layer. Improved immunity to radiation induced single event upset of the active devices is thereby achieved.
In Silicon On Insulator Field Effect Transistors (SOIFETs) formed in thin layers of silicon so that the depletion layer extends through the silicon film, the threshold voltage can be influenced by a back gate voltage. Some previous technical work on the radiation hardness of SOI CMOS devices has concentrated on simply testing the effects of total radiation on the devices. To reduce the charging of the insulator layer, frequently, a back gate formed on the surface of the substrate opposite the insulating layer is biased negatively during irradiation. However, this bias voltage is not used for dynamic threshold voltage adjustment.
U.S. Pat. No. 4,763,183, issued Aug. 9, 1988, to Ng et al., recognizes the possibility of controlling the threshold voltage of conventional MOSFET devices by applying an appropriate back gate voltage. However, in SOI devices, control of the threshold voltage by such means is dismissed as being impractical because the insulating region prevents the conduction of charge from the active region into the substrate. Instead, NG et al. modify the structure of the SOI device so as to include a conductive pathway extending from the active volume through the insulating region and into the substrate. Then control of the threshold voltage may be effected by application of a voltage to the substrate.
The temperature stabilization of threshold voltage with back gate bias is mentioned in a publication entitled "High Temperature Microelectronics - Expanding the Applications for Smart Sensors" by Brown et al., 1987 IEDM meeting sponsored by the IEEE in Washington, D.C., pp., 274-277. The technique is considered feasible because threshold voltage is a decreasing function of temperature and a increasing function of back gate bias. Brown et al. applied the bias voltage to the body of the transistor to compensate for threshold shifts due to temperature variations.
U.S. Pat. No. 4,484,076, issued Nov. 20, 1984, to I. Thomson for "Direct Reading Dosimeter" discloses a dosimeter for measuring total radiation dose in which the radiation induced change in threshold voltage of a MOSFET is measured to determine the radiation dose rate and total radiation dosage. The source of the sensor MOSFET is connected to a constant current supply, the drain thereof is grounded, and means are provided for periodically switching the gate of the sensor between ground and +V. During the time the gate of the sensor is grounded, the source to drain voltage of the sensor is a measure of the threshold voltage. The source-drain voltage of the sensor is amplified in an operational amplifier and then either applied directly to an indicator or applied to a differentiating amplifier to determine dose rate. Thomson does not recognize that the threshold voltage in a thin film SOI device can be influenced by a back gate voltage and does not utilize the output of the sensor FET, nor the outputs of the amplifiers connected thereto, to generate a back gate voltage to compensate for radiation induced threshold voltage shifts.
The threshold voltage of Silicon on Insulator Field Effect Transistors (SOI FETs) may be described by an analytical formula contained in an abstract by Caviglia et al, entitled "Threshold Voltage in Very Thin SOIFETs", 1987 IEEE SOS/SOI Technology Workshop, Durango, Colo. Oct. 6-8, 1987.